Though I still don't know why Lucci would be an exception intrinsically (considering this haki/not haki discussion). Using a zooan fruit power should make zilch difference in that sense.
Because it has claws.
Luffy is immune to bludgeoning attacks. He can still be cut and pierced.
A normal Shigan won't effect Luffy, but a Shigan done with a leopard claw will.